I forgot to wear it the other night, and my AHI increased a lot. And it's not just the AHI. It seems that when I tuck my chin (or whatever it is I'm doing with my neck) at night, my airway becomes so blocked that no amount of pressure will open the airway. This results in the CPAP increasing to max pressure, which only results in the mask leaking a lot and waking me up. Why I wake up from a blast of air and not from being unable to breathe in my sleep is one of the wonders of physiology!
Anyway, when I wear the collar, My pressure barely increases. When I don't, I have OAs, LLs, and plenty of awakenings.
Screen shots below show the night before being collar-less and the collar-less night, followed by the same pairing from another night I forgot the collar. The collared nights are reflective of most of my nights with the collar.
I Purchased a VELPEAU collar from Amazon, and I have no complaints. I'm sure most will work fine. I wear it quite loose. When I'm in bed, the back of the collar just melds into the pillow so I can't feel it, and the front is loose so that it's not even touching my neck really. It's only there to prevent my neck from tucking, and it is not uncomfortable.
If you're having a problem with mega-increases in pressure that give you large leaks, give a collar a try.